This application is based on Japanese Patent Application No. 2000-071675 filed Mar. 15, 2000, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a method of producing a hollow piston used for a compressor. More particularly, the invention is concerned with a method of producing the hollow piston from a blank which is formed by forging.
2. Discussion of the Related Art
It is desirable to reduce the weight of a piston used for a compressor, since the piston is reciprocated in a cylinder bore of the compressor. For reducing the weight of the piston, the piston has been made hollow. In general, the piston used for the compressor includes a head portion which is slidably fitted in the cylinder bore of the compressor, and an engaging portion which engages a reciprocating drive device for reciprocating the piston. The head portion of such a piston is generally made hollow. The piston having the hollow head portion is produced by preparing a cylindrical body member having an open end and a closed end and including a hollow cylindrical section and a bottom portion, and fixing a closure member to the cylindrical body member for closing its open end. In the piston having the hollow head portion, the engaging portion is formed integrally with the cylindrical body member as disclosed in JP-A-11-294320, or the engaging portion is formed integrally with the closure member as disclosed in JP-A-11-303747.
It is known that a blank for the piston having the hollow head portion is formed by casting or forging. The piston blank wherein the engaging portion is formed integrally with the cylindrical body member cannot be produced by forging. Such a piston blank can be produced by casting. Where the piston blank has the engaging portion integrally with the closure member, the weight of the closure member can be effectively reduced by forming a recess at one of opposite ends of the closure member which is remote from the engaging portion. However, it has been impossible to form this recess by forging.
In general, the piston blank formed by forging has a higher degree of strength than that formed by casting, since the material of the forged piston blank has a higher strength and the forged blank does not have blow holes or shrinkage cavities. Where the piston blank is formed by cold forging, the draft can be made smaller than where the piston blank is formed by casting. Accordingly, the weight of the hollow piston can be easily reduced where the piston is produced from the forged blank. Where the piston blank is formed by casting of an aluminum alloy, a gas is likely to be trapped in the material during the casting. In welding the cylindrical body member and the closure member together, the trapped gas expands to cause a protrusion or recess to be formed in the welded portion of the two members, resulting in reduced bonding strength or deteriorated appearance at the welded portion. In contrast, the piston blank formed by forging avoids such problems. Although it is desirable to form the piston blank by forging, the configuration of the piston blank is inevitably limited when the piston blank is formed by forging.
It is therefore an object of the present invention to produce a piston blank by forging, with an increased degree of freedom in configuration of the piston blank.
The object indicated above may be achieved according to any one of the following forms or modes of the present invention, each of which is numbered like the appended claims and depend from the other form or forms, where appropriate, to indicate and clarify possible combinations of technical features of the present invention, for easier understanding of the invention. It is to be understood that the present invention is not limited to the technical features and their combinations described below. It is also to be understood that any technical feature described below in combination with other technical features may be a subject matter of the present invention, independently of those other technical features.
(1) A method of producing a hollow piston for a compressor, the piston including a head portion which is slidably fitted in a cylinder bore of the compressor, and an engaging portion which engages a reciprocating drive device of the compressor for reciprocating the piston, at least the head portion of the piston being hollow, the method comprising a step of preparing at least one piston blank by forging, which piston blank includes an engaging-portion-forming section which provides the engaging portion of the piston, and a head-portion-forming section which provides at least a part of the head portion of the piston, the at least one piston blank being prepared by a two-axes forging process which uses a forging die assembly including a set of two first dies which are movable relative to each other along a first axis which is one of two mutually perpendicular axes, and at least one second die which is movable along a second axis which is the other of the two mutually perpendicular axes.
The method of producing the piston according to the above mode (1) of this invention assures a higher degree of freedom in the configuration of the piston blank to be produced than the conventional method which uses a forging die assembly having forging dies which are movable along only one axis, so that the hollow piston having a reduced weight can be more easily produced according to the present method.
(2) A method according to the above mode (1), wherein the first axis along which the two first dies are moved is perpendicular to an axial direction of the piston blank, while the second axis along which the at least one second die is moved is parallel to the axial direction.
The method according to the above mode (2) is effective to produce the piston blank having concave and convex portions formed or extending in its two mutually perpendicular axes, assuring an improved degree of freedom in the configuration of the piton blank to be produced. Further, the present method permits an easy manufacture of the piston blank having a reduced weight and high strength or durability. Depending upon the configuration of the piston blank to be produced, the two first dies may be moved in the axial direction of the piston blank and the at least one second die may be moved in a direction which is perpendicular to the axial direction of the piston blank.
(3) A method according to the above mode (2), wherein the at least one second die consists of at least one side punch, and the step of preparing the piston blank comprises a step of forming, by forging, the head-portion-forming section by inserting the at least one side punch into an intermediate blank which has been formed by forming the engaging-portion-forming section by forging with the two first dies.
In the method according to the above mode (3), the relative movement of the two first dies along the above-indicated first axis which is perpendicular to the axial direction of the piston blank may take place concurrently with the movement of the side punch along the second axis which is parallel to the axial direction of the piston blank. The method according to the above mode (3), however, permits easy forging of the piston blank having high dimensional and configurational accuracy.
(4) A method according to the above mode (3), wherein the at least one side punch includes a cylindrical portion, and a protruding portion which protrudes from a front end face of the cylindrical portion in the axial direction thereof and which has a non-circular shape in transverse cross section.
In the method according to the above mode (4), there is formed a recess in a part of the head-portion-forming section of the piston blank by the protruding portion of the side punch, so that the piston to be produced has a reduced weight. Although the recess may be formed by a cutting operation, the present method does not require any additional step of cutting the recess. Further, it is in general cumbersome or almost impossible to form a recess having a non-circular shape in transverse cross section by a cutting operation. In contrast, the present method permits the recess having the non-circular shape in transverse cross section to be formed by the protruding portion of the side punch constructed as described above, without the additional step of cutting.
(5) A method according to the above mode (3) or (4), wherein the at least one side punch includes a cylindrical portion, and a protruding portion which protrudes from a front end face of the cylindrical portion and which is offset from the centerline of the cylindrical portion.
The method according to the above mode (5) is effective to reduce the weight of the piston to be produced, and eliminates the additional step of cutting the recess. In general, it is cumbersome to cut the recess in a portion of the head-portion-forming section of the piston blank, which portion is offset from the centerline of the head-portion-forming section. The present method wherein the side punch has the protruding portion formed as described above permits easy formation of the recess in the corresponding portion of the head-portion-forming section of the piston blank.
(6) A method according to the any one of the above modes (3)-(5), wherein the head-portion-forming section includes a cylindrical body portion which has a hollow cylindrical section having an open end and a closed end, and a bottom portion which defines the closed end, and the at least one side punch includes a cylindrical portion having an outer circumferential surface for forming an inner circumferential surface of the cylindrical body portion, and an annular shoulder surface which extends radially outwardly from the outer circumferential surface of the cylindrical portion, for forming an annular end face of the hollow cylindrical section, which end face is remote from the bottom portion.
Where the side punch is constructed to form the annular end face of the hollow cylindrical section which is remote from the bottom portion, as well as the inner circumferential surface of the cylindrical body portion, according to the above mode (6) of the invention, the piston blank does not surfer from any burrs which would otherwise be formed on the end face of the hollow cylindrical section, for thereby eliminating an additional step of removing the burrs. In general, the closure member is fixed to the cylindrical body portion for closing its open end, by welding, bonding or friction welding, with the abutting surface of the closure member being held in close contact with the end face of the cylindrical body portion. If the bur were left on the end face, the abutting surface would not be held in close contact with the end face in the presence of the burr. Prior to the bonding of the abutting surface and the end face together, a machining operation may be effected on the end face to remove the burr therefrom. The present method, however, permits the two members to be fixed together without any clearance formed between the abutting surface and the end face while eliminating the additional step of removing the burr. Accordingly, the present arrangement does not require the machining operation on the end face of the hollow cylindrical section, or at least eliminates the step of removing the burr from the end face.
(7) A method according to the above mode (6), wherein the annular shoulder surface of the at least one side punch has an outside diameter which is larger than that of the annular end face of the hollow cylindrical section.
The annular shoulder surface of the side punch may have an outside diameter which is equal to that of the annular end face of the hollow cylindrical section. In this case, the burr is formed at the radially outer end of the annular end face such that the burr extends therefrom in the axial direction of the cylindrical body portion, or in a direction which is more or less parallel to the axial direction. In this case, the closure member for closing the open end of the cylindrical body portion can be held in close contact at its abutting surface with the annular end face of the hollow cylindrical section without removing the burr from the radially outer end of the annular end face, provided that the abutting surface of the closure member has an outside diameter which is smaller than that of the annular end face. In contrast, where the annular shoulder surface of the side punch has an outside diameter which is larger than that of the annular end face of the hollow cylindrical section according to the above mode (7) of the invention, the burr is formed at the radially outer edge of the annular end face such that the burr extends outwards in a direction parallel to the annular end face, that is, radially outwardly of the hollow cylindrical section. Accordingly, the outside diameter of the abutting surface of the closure member need not be made smaller than that of the annular end face of the hollow cylindrical section in this arrangement. For instance, the outside diameters of the abutting surface of the closure member and the annular end face of the hollow cylindrical section can be made equal to each other.
(8) A method according to the above mode (7), further comprising a step of fixing a closing member to the cylindrical body portion to close the open end of the hollow cylindrical section of the cylindrical body portion, such that the annular end face of the hollow cylindrical section is held in close contact with an end face of the closing member.
(9) A method according to the above mode (8), wherein the closing member includes a fitting portion which extends from the end face thereof, the closing member being fixed to the cylindrical body portion such that the end face of the closing member is held in close contact with the annular end face of the hollow cylindrical section of the cylindrical body member, and such that the fitting portion of the closing member is inserted into the open end of the hollow cylindrical section of the cylindrical body portion.
(10) A method according to any one of the above modes (2)-(9), wherein the engaging portion includes a pair of parallel arm sections which engage opposite surfaces of a swash plate of the reciprocating drive device at a radially outer portion thereof through a pair of shoes, and a base section which connects proximal ends of the arm sections, and the two first, dies are movable toward and away from each other in opposite directions parallel to a direction in which the arm sections extend from the base section.
(11) A method according to the above mode (10), wherein one of the two first dies is a movable die having a forming surface which gives the arm sections and which is movable toward and away from the other of the two first dies which is a stationary die.
(12) A method according to any one of the above modes (1)-(11), wherein the head-portion-forming section which provides at least a part of the head portion of the piston is formed by forging as a cylindrical body portion that gives a principal part of the head portion of the piston.
(13) A method according to any one of the above modes (1)-(11), wherein the head-portion-forming section which provides at least a part of the head portion of the piston is formed by forging as a closure member which closes an open end of a cylindrical body portion that gives a principal part of the head portion of the piston.
(14) A method of producing a blank for a hollow piston used for a compressor, the piston including a head portion which is slidably fitted in a cylinder bore of the compressor, and an engaging portion which engages a reciprocating drive device of the compressor for reciprocating the piston, at least the head portion of the piston being hollow, the method comprising a step of preparing at least one piston blank including an engaging-portion-forming section which provides the engaging portion of the piston, and a head-portion-forming section which provides at least a part of the head portion of the piston, the at least one piston blank being prepared by a two-axes forging process which uses a forging die assembly including a set of two first dies which are movable relative to each other along a first axis which is one of two mutually perpendicular axes, and at least one second die which is movable along a second axis which is the other of the two mutually perpendicular axes.
The method according to the above mode (14) of this invention may have any one of the features included in the above modes (2)-(13).
(15) An apparatus for producing a blank for a hollow piston used for a compressor, the piston including a head portion which is slidably fitted in a cylinder bore of the compressor, and an engaging portion which engages a reciprocating drive device of the compressor for reciprocating the piston, at least the head portion of the piston being hollow, the apparatus comprising a forging die assembly which includes a set of two first dies which are movable relative to each other along a first axis which is one of two mutually perpendicular axes of the blank, and at least one second die which is movable along a second axis which is the other of the two mutually perpendicular axes.
(16) An apparatus according to the above mode (15), wherein the first axis along which the two first dies are moved is perpendicular to an axial direction of the blank, while the second axis along which the at least one second die is moved is parallel to the axial direction, the at least one second die consisting of at least one side punch.
The apparatus according to the above mode (16) of this invention has any one of the features included in the above modes (3)-(13).
(17) An apparatus according to the above mode (16), wherein the forging die assembly further includes a motion converting device for converting a relative movement of the two first dies along the first axis into a movement of the at least one side punch along the second axis.
(18) An apparatus according to the above mode (17), wherein the motion converting device comprises a cam device including a drive cam and a driven cam.
(19) An apparatus according to the above mode (17) or (18), wherein the forging die assembly further comprises at least one of a first die holder for holding one of the two first dies and a second die holder for holding the other of the two first dies, and a hydraulically operated cylinder which is located between the at least one of the first and second die holders and a corresponding one of the two first dies which is held by the at least one die holder, for moving the at least one die holder and the corresponding one of the two first dies relative to each other in a direction parallel to the first axis along which the two first dies are movable relative to each other.
(20) An apparatus according to any one of the above modes 16-19, wherein the forging die assembly further comprises: a first and a second die holder for holding one and the other of the two first dies, respectively, and a motion converting mechanism for converting a relative movement of the first and second die holders toward each other along the first axis, successively into: (a) a movement for positioning a forging blank from which the piston blank is to be produced, with respect to the second axis, by activating the at least one side punch with a force which does not cause deformation of the forging blank; (b) a movement for forging the forging blank with the two first dies to form an intermediate blank, and (c) a movement for inserting the at least one side punch into the intermediate blank so as to form the blank for the hollow piston.